There are already known various constructions of surface acoustic wave devices, among them such using quarter wavelength electrodes or eighth wavelength (split) electrodes. The electrodes of such SAW devices may be situated on a surface region of a dielectric substrate, or they may at least partially penetrate below such surface region and into the substrate proper.
It is known to change the operating frequency of a SAW device (see an article by W. J. Tanski published in Applied Physics Letters, 39(1) (1981) and entitled "Surface Acoustic Wave Frequency Trimming of Resonant and Traveling Wave devices on Quartz"), in an iterative manner, from that at which the device operates as manufactured to that at which the device is supposed to operate when in use. The frequency trimming technique disclosed in this article involves etching of the quartz substrate between the metallic electrodes to a certain depth which depends on the extent of deviation of the actual operating frequency from that desired. This etching results in an increase in the size of the steps between the electrodes and the quartz, with attendant velocity decrease which, in turn, causes the operating frequency to go down. An additional result of the step size increase is an increase in the transducer reflectivity when the SAW device is a resonator.
While this article mentions that it is usually desirable to reduce the SAW reflectivity of the transducers in resonators formed on quartz, not only does it fail to specifically disclose how such reflectivity reduction could be achieved, but it also proposes deliberately to increase the transducer reflectivity in order to accomplish the desired goal, that is, to trim the resonance frequency downward. Furthermore, in the SAW device constructions disclosed in this article, reflectivity can never be reduced by etching, no matter how great the amount of etching may be.
Now, SAW filters made on quartz, especially those that are to be used in the VHF, UHF and L band frequency ranges, must have the mechanical or acoustic reflectivity of the electro-acoustic transducer electrodes reduced to a negligible amount in order to ensure the desirable response in a relatively easily reproducible manner. An approach that is often used to reduce electrode reflectivity is to employ eighth wavelength (or split) electrodes. However, this approach works to satisfaction only in the frequency range up to about 400 MHz; above this frequency range, split electrodes cannot, for all intents and purposes, be employed inasmuch as they would require linewidths between 0.25 and 1 micron, which is at if not beyond the limit of what can be reliably achieved, at least at the present time.
The acoustic reflectivity of SAW device electrodes, especially of quarter wavelength aluminum electrodes on quartz, can be reduced by recessing the electrodes in the substrate in such a manner that a groove is first etched in the substrate at the contemplated location of the respective electrode, and then this groove is filled with the electrode metal. This technique is quite effective, but the quality of the result is highly dependent on the accuracy and reproducibility of the fabrication procedures. Experience has shown that the yield of high quality recessed electrode SAW filters obtained when using the aforementioned fabrication technique is quite low.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a method of making SAW devices, which method does not have the disadvantages of the known methods of this kind.
Still another object of the present invention is to develop the method of the type here under consideration in such a manner as to dramatically increase the yield of high-quality SAW devices from a batch of such devices obtained by following the recessed-electrode fabrication technique.
It is yet another object of the present invention to devise a method of the above type which renders it possible to keep the difference between the actual and the desired acoustic reflectivity in SAW devices to a minimum.
A concomitant object of the present invention is to design the SAW device in such a manner as to lend itself to a performance of the above method thereon while still being relatively simple in construction, inexpensive to manufacture, easy to use, and at least as reliable in operation as prior-art SAW devices.